1) Which is the commonly used universal gate and why (between NAND n NOR) ?
2)What is the basic differences between C and VHDL ?
3)How does the device scaling change for NOT,NAND,NOR ?
4)What is set-up time and hold-time in case of flip flops ?
5)Why is BJT more faster than MOS devices ?

1) Which is the commonly used universal gate and why (between NAND n NOR) ?
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Althought it is possible to build any logic circuit using only NAND or only NOR gates, both NAND and NOR are used for reasons of simplicity. Basically you would need 4 NAND gates to mimic a NOR gate and vice versa. So, in order to use less transistors, both NAND and NOR gates are used in logic chips (keep in mind that a NOT gate is a NAND or NOR gate with one input).

VLSI circuits are generally implemented using CMOS technology. The most common gate type is NAND because the pMOS pull-ups are in parallel as opposed to series in NOR gates. Why is this a problem? Because pMOS transistors are slower than nMOS transistors due to the relative mobility of electrons and holes.

C is a procedural software programming language for writing programmes. VHDL is a hardware description language decrbing hardware structure or behaviour. A quick search on the net should give you more specifics on the differences between these general classes of programming languages.

The data input to a FF must be stable for some period of time around the clock-edge of the FF if it is to be reliably sampled. The input must settle some time (this is the set-up time) before the edge of the clock, and should not change till some time after the clock-edge (this is the hold time).

In VLSI (CMOS) circuits, this is because the output form one logic gate drives the input to another logic gate. In CMOS in the inputs to a logic gate drives the gate of two MOSFETs - this increases the associated RC time constant resulting in the general operation being slower.

What you had were not doubts -- they were questions. This mistake is so common I thought I should remind you. A question is something you ask when you don't know the answer. A doubt is something that you have when you know the answer, but you are uncertain or unsure of your belief.